Gravitational Lensing

This Photonic Crystal Bends Light Like a Black Hole

One of the first observational tests of general relativity was that the path of light bends in the presence of mass. Not only refracts the way light changes direction as it enters glass or other transparent materials, but bends along a curved bath. This effect is central to a range of physical phenomena, from black holes to gravitational lensing to observations of dark matter. But because the effect is so tiny on human scales, we can’t study it easily in the lab. That could change in the future thanks to a new discovery using distorted photonic crystals.

Photonic crystals are materials with a periodic refractive index on nanometer scales. They occur naturally in things such as opals and the wings of some species of butterflies, which gives them their colorful pearlescent rippling effect. They’ve been known since the 1800s, but in the late 1980s, we began to be able to make simple photonic crystals, and research on the materials really started to take off.

Fiber optics and other advanced optical materials ushered in the field of photonics, where we can now start to make photonic crystal materials with very specific properties, such as tuning them to be sensitive to specific wavelengths or focusing light more effectively. This new research focuses on a type of material known as distorted photonic crystals.

Bending light with a distorted photonic crystal. Credit: K. Kitamura

Normally you wouldn’t want your crystal to have any distortions. The more consistent you can make your material, the more uniformly light will behave while passing through it. But in this case, the team was able to gradually deform the spacing of the crystal lattice. This meant that the periodic refractive index shifts gradually as you move through the material. For light, this means the amount of refraction gradually varies, just as it does for light passing near a massive body such as a black hole. The result is that light follows the same kind of curved path as gravitationally lensed light.

The authors call this effect pseudogravity, and it could be used to simulate the effects of general relativity. You could imagine being able to create photonic crystals that simulate the lensing effects of galaxies, or even simulations of a black hole’s event horizon. If we can make distorted crystals with the right properties, we can do all kinds of pseudogravity experiments.

While pseudogravity makes for great headlines, the early uses for distorted photonic crystals will be in optical communications and optical computing. The crystals can deflect light paths without any significant loss of intensity or signal, which will be a powerful tool for things such as ultra-high-speed internet and the next generation of mobile communication.

This means when we do get around to doing pseudogravity experiments, we’ll be able to communicate the results with incredible speed and efficiency.

Reference: Nanjyo, Kanji, et al. “Deflection of electromagnetic waves by pseudogravity in distorted photonic crystals.” Physical Review A 108.3 (2023): 033522.

Brian Koberlein

Brian Koberlein is an astrophysicist and science writer with the National Radio Astronomy Observatory. He writes about astronomy and astrophysics on his blog. You can follow him on YouTube, and on Twitter @BrianKoberlein.

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